directly assessing the biological integrity of a site and the
only method that is sensitive to toxicological
influences and habitat degradation resulting from changes in land use or
such instream disturbances as floods, navigation improvements, or
substrate instability.

The distribution of benthic
invertebrate species responds to natural and anthropogenic influences.
Natural changes in physical and chemical conditions occur along the
longitudinal axis of the river (Vannote and others, 1980), altering
environmental variables (for example, riparian conditions, thermal
regimes, discharge patterns, light penetration, channel gradients,
sediment conditions, water and sediment chemistry) and causing benthic
invertebrate communities to change. In addition, each location along
the river continuum will contain a variety of habitats, such as riffles,
pools, sloughs, bars, and backwaters, that differ in respect to substrate
type and stability, current velocity, and water depth. Therefore, each
location in the river has a range of natural conditions that, when
coupled with environmental requirements of the invertebrate species,
determine whether a given organism can live in a particular habitat at a
particular point along the river continuum.

These patterns of species
distribution are affected by natural and anthropogenic influences that
alter the landscape (for example, wild fires, logging, earthquakes,
agriculture, volcanic eruptions, and urbanization), modify hydrologic
conditions (changes in evapotranspiration and runoff or construction of
reservoirs and irrigation diversions), alter habitats (snagging
operations, channel dredging, sedimentation, hurricanes), or add chemicals
that are toxic or that elevate nutrient or organic loads. The challenge
of ecological surveys in the NAWQA Program is to separate changes caused
by natural and anthropogenic factors and relate them to water quality.
This is accomplished by comparing distributions of organisms among sites
that vary in natural and anthropogenic influences, including relatively
pristine streams, and relating patterns of distribution to patterns of
physical and chemical factors.

The sampling methods and procedures presented here are
intended to give guidance to study-unit biologists collecting benthic
invertebrates as part of the USGS's NAWQA Program. Various
sample-collection techniques, equipment, and data forms are presented for
use at basic fixed sampling sites. These methods and techniques can be
adapted for use in other components of the NAWQA Program, such as synoptic
and case study sampling, or where needed in other programs of the USGS's
Water Resources Division. Additional discussions and descriptions of
benthic invertebrate sampling devices and methods are reported in Hynes
(1970), Mason (1978), Rosenberg (1978), Adamus (1984), Merritt and Cummins
(1984), Britton and Greeson (1988), and Klemm and others (1990).